How to download and install an add-on hosted on github properly?
Option 1 - Release version
Before downloading the entire repository using the 'Clone or download' button, make sure there is no actual release (In most cases there is, to keep development organized):


If so, click on Releases Tab and download the latest release (zip or single py file):


Option 2 - init.py folder (Clone or download)
There are lazy authors so in some rare cases the 'Clone or download' is the only option. Search for __init__.py in the repository and copy the folder containing this file to your Add-on directory:

Linux: $HOME/.config/blender/&lt;version&gt;/
OSX: /Library/Application Support/Blender/&lt;version&gt;/
Windows: %USERPROFILE%\AppData\Roaming\Blender Foundation\Blender\&lt;version&gt;\

You can get also get the folder per machine using the python console: bpy.utils.script_path_user() and / or bpy.utils.script_paths(&quot;addons&quot;)
Further reading: Addons: Only some .zips will install